A client application can request cancellation of a command
that is still being processed by the server, using the functions
described in this section.

PQgetCancel

Creates a data structure containing the information
needed to cancel a command issued through a particular
database connection.

PGcancel *PQgetCancel(PGconn *conn);

PQgetCancel creates a
PGcancel object given a
PGconn connection object. It
will return NULL if the given
conn is NULL or an invalid connection. The PGcancel object is an opaque structure
that is not meant to be accessed directly by the
application; it can only be passed to PQcancel or PQfreeCancel.

PQfreeCancel

Frees a data structure created by PQgetCancel.

void PQfreeCancel(PGcancel *cancel);

PQfreeCancel frees a data
object previously created by PQgetCancel.

PQcancel

Requests that the server abandon processing of the
current command.

int PQcancel(PGcancel *cancel, char *errbuf, int errbufsize);

The return value is 1 if the cancel request was
successfully dispatched and 0 if not. If not, errbuf is filled with an explanatory error
message. errbuf must be a char
array of size errbufsize (the
recommended size is 256 bytes).

Successful dispatch is no guarantee that the request
will have any effect, however. If the cancellation is
effective, the current command will terminate early and
return an error result. If the cancellation fails (say,
because the server was already done processing the
command), then there will be no visible result at all.

PQcancel can safely be
invoked from a signal handler, if the errbuf is a local variable in the signal
handler. The PGcancel object is
read-only as far as PQcancel
is concerned, so it can also be invoked from a thread that
is separate from the one manipulating the PGconn object.

PQrequestCancel

PQrequestCancel is a
deprecated variant of PQcancel.

int PQrequestCancel(PGconn *conn);

Requests that the server abandon processing of the
current command. It operates directly on the PGconn object, and in case of failure
stores the error message in the PGconn object (whence it can be retrieved
by PQerrorMessage). Although
the functionality is the same, this approach creates
hazards for multiple-thread programs and signal handlers,
since it is possible that overwriting the PGconn's error message will mess up the
operation currently in progress on the connection.

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